I think it's in case Google's auth (or even Google itself!) goes down—Facebook's was down the other day, and if you don't provide alternative sign-in mechanisms, users can't sign in.
Sure, but as a site you should either depend on 3rd-party auth and deal with the possibility of downtime, or be up-front about wanting your own credentials. To pretend like FB or Google login will work and then demanding a user/pass is deceptive and really, really annoying.
